Solve {l}{x=3y}{2(x+y)+2x=280}

$$\left\{ \begin{array} { l } { x = 3 y } \\ { 2 ( x + y ) + 2 x = 280 } \end{array} \right.$$

Solve for x, y

$$x=60$$

$$y=20$$

Steps Using Substitution

Consider the first equation. Subtract 3y from both sides.

$$x-3y=0$$

Consider the second equation. Use the distributive property to multiply 2 by x+y.

$$2x+2y+2x=280$$

Combine 2x and 2x to get 4x.

$$4x+2y=280$$

To solve a pair of equations using substitution, first solve one of the equations for one of the variables. Then substitute the result for that variable in the other equation.

$$x-3y=0,4x+2y=280$$

Choose one of the equations and solve it for x by isolating x on the left hand side of the equal sign.

$$x-3y=0$$

Add 3y to both sides of the equation.

$$x=3y$$

Substitute 3y for x in the other equation, 4x+2y=280.

$$4\times 3y+2y=280$$

Multiply 4 times 3y.

$$12y+2y=280$$

Add 12y to 2y.

$$14y=280$$

Divide both sides by 14.

$$y=20$$

Substitute 20 for y in x=3y. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=3\times 20$$

Multiply 3 times 20.

$$x=60$$

The system is now solved.

$$x=60,y=20$$

Steps Using Matrices

Consider the first equation. Subtract 3y from both sides.

$$x-3y=0$$

Consider the second equation. Use the distributive property to multiply 2 by x+y.

$$2x+2y+2x=280$$

Combine 2x and 2x to get 4x.

$$4x+2y=280$$

Put the equations in standard form and then use matrices to solve the system of equations.

$$x-3y=0,4x+2y=280$$

Write the equations in matrix form.

$$\left(\begin{matrix}1&-3\\4&2\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}0\\280\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}1&-3\\4&2\end{matrix}\right).

$$inverse(\left(\begin{matrix}1&-3\\4&2\end{matrix}\right))\left(\begin{matrix}1&-3\\4&2\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&-3\\4&2\end{matrix}\right))\left(\begin{matrix}0\\280\end{matrix}\right)$$

The product of a matrix and its inverse is the identity matrix.

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&-3\\4&2\end{matrix}\right))\left(\begin{matrix}0\\280\end{matrix}\right)$$

Multiply the matrices on the left hand side of the equal sign.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&-3\\4&2\end{matrix}\right))\left(\begin{matrix}0\\280\end{matrix}\right)$$

For the 2\times 2 matrix \left(\begin{matrix}a&b\\c&d\end{matrix}\right), the inverse matrix is \left(\begin{matrix}\frac{d}{ad-bc}&\frac{-b}{ad-bc}\\\frac{-c}{ad-bc}&\frac{a}{ad-bc}\end{matrix}\right), so the matrix equation can be rewritten as a matrix multiplication problem.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{2}{2-\left(-3\times 4\right)}&-\frac{-3}{2-\left(-3\times 4\right)}\\-\frac{4}{2-\left(-3\times 4\right)}&\frac{1}{2-\left(-3\times 4\right)}\end{matrix}\right)\left(\begin{matrix}0\\280\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{7}&\frac{3}{14}\\-\frac{2}{7}&\frac{1}{14}\end{matrix}\right)\left(\begin{matrix}0\\280\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{3}{14}\times 280\\\frac{1}{14}\times 280\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}60\\20\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=60,y=20$$

Steps Using Elimination

Consider the first equation. Subtract 3y from both sides.

$$x-3y=0$$

Consider the second equation. Use the distributive property to multiply 2 by x+y.

$$2x+2y+2x=280$$

Combine 2x and 2x to get 4x.

$$4x+2y=280$$

In order to solve by elimination, coefficients of one of the variables must be the same in both equations so that the variable will cancel out when one equation is subtracted from the other.

$$x-3y=0,4x+2y=280$$

To make x and 4x equal, multiply all terms on each side of the first equation by 4 and all terms on each side of the second by 1.

$$4x+4\left(-3\right)y=0,4x+2y=280$$

Simplify.

$$4x-12y=0,4x+2y=280$$

Subtract 4x+2y=280 from 4x-12y=0 by subtracting like terms on each side of the equal sign.

$$4x-4x-12y-2y=-280$$

Add 4x to -4x. Terms 4x and -4x cancel out, leaving an equation with only one variable that can be solved.

$$-12y-2y=-280$$

Add -12y to -2y.

$$-14y=-280$$

Divide both sides by -14.

$$y=20$$

Substitute 20 for y in 4x+2y=280. Because the resulting equation contains only one variable, you can solve for x directly.

$$4x+2\times 20=280$$

Multiply 2 times 20.

$$4x+40=280$$

Subtract 40 from both sides of the equation.

$$4x=240$$

Divide both sides by 4.

$$x=60$$

The system is now solved.

$$x=60,y=20$$